Recently, radiation therapy to cause tumor cell necrosis by irradiation with various radioactive rays has been widely performed. The radioactive rays used in the art include not only an X ray that is most widely used but also particle beams including a proton beam. Therapy using an X ray or particle beams is also performed. Radiation therapy is usually performed by four steps of diagnosis, treatment plan, treatment, and follow-up examinations. In each of the steps, image guided radiation therapy (IGRT) is increasingly performed using images or image processing technique aiming at more accurate treatment. In the step of diagnosis, for example, with more variations and higher performance of diagnostic devices, structural images of magnetic resonance (MR) images or functional images of positron emission tomography (PET) images are used for determination of treatment regions in treatment plans in addition to X ray computed tomography (CT) images that has been conventionally used for treatment plans.
Meanwhile, one of important processes in a treatment is positioning of a bed. This is to match the current bed position to a position in the treatment plan upon setting up a patient immediately before the treatment. In radiation therapy in general, an affected part is irradiated with radioactive rays multiple times (depends on a treatment part, for example several tens of times) and thus bed positioning is performed each time upon irradiation. Specifically, it is a process of calculating or determining a shift between a position of irradiation target determined in the treatment plan and a current position of the irradiation target on a therapeutic bed (hereinafter abbreviated as “bed”) by comparison, by an engineer or doctor, between a digital reconstructed radiograph (DRR) image output from a treatment planning device and an X ray image (digital radiograph (DR) image) photographed using an X ray imaging device while a patient is lying on the bed before irradiation of radioactive rays, thereby obtaining a displacement amount of the bed such that the images of the two types match with each other, and thereby moving the bed.
In this manner, for treatment plans of radiation therapy, treatment, or bed positioning, methods for aligning corresponding positions of a plurality of images include a registration technique. Conventionally, rigid registration has been the mainstream where a displacement amount is obtained with an assumption that an imaging object in an image is not deformed. In radiation therapy, however, an imaging object is human and subjected to non-rigid deformation. Therefore, non-rigid registration corresponding to such deformation started to be in use in CT images for treatment plans. One of such a technique, PTL 1 proposes a method for deforming a region of interest set in a treatment plan and recognizing a target.